It doesn't define it as deallocate to the OS. Deallocating it to the OSis counterproductive to most paging/virtual memory OS's as it fragmentsthe virtual address space maps quite badly. For some cases the standardmalloc behaviour is not ideal, but very few. You can use other malloclibraries for that. Another example where I use different malloc librariesis the multiuser game I wrote - here because the system malloc has a higheroverhead and I allocate vast numbers of tiny objects. The malloc I usealso knows about the game malloc order and thus optimises for pagelocality.